A high pressure discharge lamp includes an arc tube in which a pair of electrodes are disposed in opposition to each other, and is used as a light source in a projection-type image display apparatus such as a liquid crystal projector.
Normally, such a high pressure discharge lamp is lit by a method of lighting the lamp at a constant current value in an initial stage, and thereafter changing to constant power control by supplying a predetermined power (rated power) to the lamp (e.g., see patent citation 1).
There is demand for increased brightness (illuminance on the screen, which is hereinafter referred to as simply “illuminance”) in this type of projection-type image display apparatus, and therefore various improvements in the high pressure discharge lamp included therein are required.
One example of an improvement involves the configuration of the electrodes. Specifically, the tip portions of the electrodes are formed into configurations from substantially hemispherical to substantially conical (e.g., see patent citation 2). Light beams that irradiate from the arc between the electrodes toward the electrodes are blocked by the electrodes and cannot be emitted out of the arc tube. However, the above configurations reduce the proportion of light beams that are blocked by the electrodes, thereby increasing the amount of luminous flux that is emitted out of the arc tube, and contributing to an improvement in illuminance.
Also, another method that has been proposed involves improving illuminance by raising the amount of enclosed mercury in order to increase the brightness of the high pressure discharge lamp itself.    Patent Citation 1: Japanese Patent Application Publication No. 2000-306687    Patent Citation 2: Japanese Patent Application Publication No. 2002-93363